There are numerous advantages to implementing a radio communication system using digital techniques. Notably, there is enhanced system capacity, reduced noise, and reduced hardware and associated power consumption. There has been proposed several digital radio communication systems. For example, there is shown and described in the commonly assigned U.S. Patent Application entitled "Multi-Channel Digital Transceiver and Method", filed on even date herewith and of which the Applicants are co-inventors, several preferred embodiments of radio communication systems implementing digital techniques.
Fundamental to the digital radio communication system is the requirement that the received analog radio signal be digitized. The well known Nyquist criteria provides that such digitization is accomplished with minimal error using an analog-to-digital converter (ADC) with a sampling rate greater than twice the bandwidth of the analog signal. In U.S. Pat. No. 5,251,218 a methodology typical of the prior art is disclosed for digitizing an analog radio frequency signal. It will be appreciated, however, where the radio signal occupies a large bandwidth, ADCs capable of operation at very high sampling rates are required. Such devices, to the extent they are available, are expensive and often suffer reduced performance, i.e., have significant distortion and increased power consumption when operated at high sampling rates.
The spectrum allocated to radio communication systems is typically large with respect to the requirements for digitizing. In some radio communication systems, however, although the desired signal occupies a large bandwidth, not all of the bandwidth is occupied by signals of interest. In cellular radio telephone communication systems, for example, the communication bandwidth is not contiguous. When radio spectrum was initially allocated for cellular radio telephone communications, two contiguous 10 Megahertz (MHz) blocks of spectrum where allocated for "A-band" operators and "B-band" operators, respectively. However, as the need arose to enhance capacity of the cellular radio communication system, additional bandwidth was required. Unfortunately, large enough blocks of bandwidth adjacent the originally allocated blocks were not available. Hence, additional 2.5 MHz blocks of bandwidth were allocated. The resulting A-band is illustrated in FIG. 2A as an 11 MHz block and a 1.5 MHz block separated by a 10 MHz block occupied by the B-band. So, while the cellular A-band and B-band each have a bandwidth of 12.5 MHz, spectrally, the entire A-band covers 22.5 MHz of bandwidth in two discontinuous portions.
In order to digitize the A-band, for example, one would need an ADC capable of operating, according to the Nyquist criteria, at least at 45 Mhz or 45 million samples per second (Ms/s), and more reliably at 56 Ms/s. Splitting the signal into smaller segments allows the use of multiple ADCs at lower sampling rates. Using multiple ADCs has the disadvantage of requiring more hardware. Furthermore, clock frequency and higher order harmonics thereof inevitably fall into the frequency band of the signal being digitized. Still another disadvantage is the amount of digital data handling required to filter, interpolate, compensate for band overlap and sum the resulting multiple digital signals.
Therefore, there is a need for a device and method for digitizing split frequency band signals which does not require high sampling rates, and does not significantly increase the amount of hardware required for the communication system. Such a device and method are provided by the present invention, and one of ordinary skill in the art will readily appreciated the many advantages and features of the present invention from the following detailed description and accompanying drawings: